Passive selective catalyst reduction (SCR) systems can be used as aftertreatment systems for lean burn spark ignition (SI)-engines. Their operation is based on the interaction between the engine, an ammonia formation catalyst (AFC), and an SCR catalyst. Under rich conditions the AFC forms ammonia, which is stored in the SCR catalyst. Under lean conditions, the SCR catalyst reduces the engine out NOx using the stored NH3.This study compared the ammonia production and response times of a standard three way catalyst (TWC) and a Pd/Al2O3 catalyst under realistic engine operating conditions. In addition, the relationships between selected engine operating parameters and ammonia formation over a TWC were investigated, considering the influence of both the chosen load point and the engine settings.Tests at the loadpoint of 1500 rpm/2.63 bar break mean effective pressure (BMEP) showed that most ammonia was formed over a TWC at a lambda of 0.93 with a 50% mass fraction burned (MFB) at 3 crank angle degrees (CAD) after top dead centre (aTDC), when valve overlap was minimised. The response time of the TWC during transitions from rich to lean operation was evaluated at various loadpoints, showing that the TWC responded most rapidly at higher loadpoints, mainly due to the higher flow through the catalyst. Furthermore it was noted that at 1750 rpm/8.00 bar BMEP, ammonia formation decreased over time due to high in-catalyst temperatures.A comparison between the TWC and Pd/Al2O3 catalyst showed that the latter produced ammonia faster than the TWC catalyst at 1500 rpm/2.63 bar BMEP. Furthermore the yield was more than 50% lower compared to the TWC.